Electrophotographic marking is a well-known method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired final image onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges to produce an electrostatic latent image of the desired image on the photoreceptor's surface. Toner particles are then deposited onto that latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, to produce the desired document. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another document.
The foregoing broadly describes a black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process each color of toner that is used to make the composite color image. By placing the various colors of toner in a superimposed registration a desired composite color image results.
One way of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is comprised of a laser light source (or sources) and a rotating polygon having a plurality of mirrored facets. The light source radiates a laser beam onto the polygon facets. That beam reflects from the facets and strikes the photoreceptor, producing a light spot. As the polygon rotates the spot traces lines, referred to as scan lines, on the photoreceptor. The direction of the sweeping spot is called the fast scan direction. By moving the photoreceptor perpendicular to the fast scan direction, as the polygon rotates the spot raster scans the photoreceptor. The direction of motion of the photoreceptor is referred to either as the slow scan direction or the process direction. During scanning, the laser beam is modulated to produce the desired latent image.
The position of the raster output scanner relative to the photoreceptor is very important. Typically, a raster output scanner is located within a special housing that attenuates noise from the rotating polygon and that prevents paper fibers, toner, and other debris from contaminating the optical components. That housing is positioned within a machine frame such that the laser beam that emerges from a housing window is directed along a scan plane. Ideally, that scan plan is perpendicular to the direction of motion of the photoreceptor. Scan lines that are not perpendicular to the direction of motion of the photoreceptor are referred to as being skewed. Furthermore, the aerial image formed by the laser beam on the photoreceptor should be properly focused. While positioning the raster output scanner relative to the photoreceptor is difficult in black only printing, when color printing using multiple raster output scanners proper positioning of the raster output scanners relative to the photoreceptor or photoreceptors becomes even more difficult. Not only must multiple raster output scanners be positioned and mounted, but also the eye's sensitivity to color imperfections mandate tight positioning tolerances.
In addition to initially positioning the raster output scanner relative to the photoreceptor, it is important that the position remains correct over time. For example, vibrations within the machine and photoreceptor motion, particularly when using a belt photoreceptor, must not cause positioning problems.
It is therefore one object of the invention to provide a system for mounting a raster output scanner relative to a photoreceptor such that the relative positioning of the elements meet tight imaging tolerances.